Why does my model overfit on pseudo-random numbers training data?
Simply said, predicting pseudo random number is just not possible for now. Pseudo random numbers generated now have a high enough "randomness" so that it cannot be predicted. Pseudo random numbers is the basis of modern cryptography which is widely used in the world wide web and more. It may be possible in the future through faster computers and stronger AI, but for now it is not possible. If you train a model to fit on pseudo random numbers, the model will just overfit and thus creating a scenario as shown in the question. The training loss will be very low while test loss will be extremely high. The model will just "remember" the training data instead of generalising to all pseudo random numbers, thus the high test loss.

Also, as a side note, loss is not represented by %, instead it is just a raw numeric value.

See this stack exchange answer for details.